Codon usage

Dear all,
I need some help concerning differences in the usage of synonymous codons
within a single gene:
I have investigated the distribution of rarely used codons in a highly
expressed larval hemolymph protein of Drosophila. I found that there is an
accumulation of non-optimal codons (according to the Drosophila codon
usage table) at the N-terminal and the C-terminal ends.
I am aware of the paper of Akashi (Genetics, 136, 927-935, 1994) providing
evidence for selection for "optimal" codons in structural important
regions. However, in this case it seems as if there were a preference of
"non-optimal" codons in some regions of the gene. How to explain this
phenomenon? Translational slowdown, e.g. for proper folding of the nascent
protein?
Any idea? Or does anybody know a relevant reference of a similar example?
Thanks, Thorsten
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Dr. Thorsten Burmester
Institut Jacques-Monod Tel: (+33) 1 44 27 40 94
CNRS et Universite Paris 7 Fax: (+33) 1 44 27 52 65
2 place Jussieu email: burmeste at ccr.jussieu.fr
F-75251 Paris CEDEX 05
France